Hydroxyls

In chemistry, hydroxyls refer to species containing the hydroxyl group or related forms. The term is used for the hydroxyl functional group in organic and biomolecules (-OH), the hydroxide ion (OH−) in inorganic salts and aqueous solutions, and the hydroxyl radical (OH•), a highly reactive transient species.

The hydroxyl group (-OH) consists of an oxygen atom bonded to hydrogen. It is highly polar and forms hydrogen bonds, raising boiling points and increasing water solubility. In organic chemistry, hydroxyl groups appear in alcohols (methanol, ethanol) and phenols, influencing acidity and reactivity.

The hydroxide ion (OH−) is the conjugate base of water. In aqueous solutions it acts as a base and a nucleophile, accepting protons to form water and engaging in substitutions. The conjugate acid of OH− is H2O, with a pKa around 15.7, indicating OH− is a relatively strong base in water.

The hydroxyl radical OH• is a highly reactive species produced by photolysis of hydrogen peroxide, ozone, or organic peroxides, and during combustion and certain environmental processes. It rapidly reacts with many organic molecules by hydrogen-atom abstraction or addition to double bonds, initiating oxidation. Because of its reactivity and short lifetime, OH• is a key oxidant in atmospheric chemistry and in laboratory advanced oxidation processes for water purification.

Hydroxyls are widespread in chemistry and biology. Hydroxyl groups are central in alcohols, sugars, amino acids, and nucleic acids, and play key roles in enzyme catalysis and metabolism. In geology and materials science, hydroxide minerals contain hydroxyl groups. Hydroxyl radicals are highly reactive oxidants with environmental and health relevance.